Evolution of the C-H aliphatic component of the interstellar matter under cosmic ray irradiation. The values of σ_d were determined from a-C:H 1 data with α = 1.0 (see Fig. 5). The black curve corresponds to an exponential decrease without residual hydrogen content taking into account (ρ/ρ_i = exp(−R_d,CR·t)). The red solid (fit 1) and dashed (fit 2) curves are obtained when the asymptotic values ρ_f/ρ_i ≠ 0 are considered. Values of ρ_f/ρ_i are determined from a-C:H 1 data and two different fits (see Fig. 6 and text for details). The dotted vertical line corresponds to a typical cloud time scale t = 3  ×  10⁷ yr. (McKee 1989; Draine 1990; Jones et al. 1994).